This invention relates to an inorganic vitreous material suitable for ion exchange, and ion exchanged products produced thereby, wherein the refractive index is changed considerably, but wherein the linear thermal expansion coefficient is not altered significantly, if at all. The glass according to this invention can be utilized as a material for gradient rods, gradient lenses, fiber optics with refractive index gradients, and other optical systems utilizing glass having refractive index gradients.
It is known that changes in concentration can be caused in glasses by altering the composition of the melt, as well as by ion exchange. Changes in properties attained thereby can generally be determined by calculations. However, this invention could not result from mere calculations.
Ion exchange processes between glass and suitable media, e.g. salt solutions, salt melts, suspensions, or solid layers applied in any desired way, are conventional. On the basis of extensively investigated diffusion processes, an exchange takes place between readily diffusing ions of the glass and those of a suitable medium. This process is, in most cases, greatly dependent on the temperature.
These ion exchange processes are frequently utilized to change the characteristics of the glass. The best-known example is the increase in strength by means of ion exchange taking place in various ways at below and above the relaxation temperature. While at lower temperatures (below a temperature corrresponding to a glass viscosity of 10.sup.14.5 poises) small ions of the glass are exchanged for larger ions from the suitable medium, thus resulting in a buildup of internal pressure in the zone affected by the ion exchange, the glass composition is altered, at higher temperatures, by the ion exchange so that the composition obtains a lower linear thermal expansion coefficient. Thus, both processes lead to the buildup of compressive stresses in the exchanged zone. A summary of these methods can be found in H. Schroder and G. Gliemeroth ("Festigkeitserhoehung von Glaesern durch Oberflaechenbehandlung" [Increasing the Strength of Glasses by Surface Treatment], "Naturwissenschaften" [Natural Sciences] 57 [1970]).
In general, the most readily diffusing ions of Li, Na, K, Rb, Cs, Tl are utilized as the diffusing ions, but considerations and experiments have already been conducted on the use of ions of Pb, Ba, Cd, Sr, Ca, Zn, Mg, Be for changing characteristics by ion exchange.
Just as the linear thermal expansion coefficient can be altered, it is also possible to change other properties of the glass in the zone subjected to the ion exchange. One possibility is the alteration of the refractive index, which is utilized especially in the field of refractive index gradient generation.
In this method, the concentration profiles produced in the glass during the course of the ion exchange are utilized. For if ions of the glass are exchanged against ions lowering the refractive index of the glass, a refractive index profile is obtained after termination of the ion exchange which is proportional to the concentration profile of the ions migrated into the glass. In the present case, the refractive index thus becomes increasingly lower with a rising concentration of migrated ions. Several examples for utilizing these refractive index profiles produced by ion exchange can be found in U.S. Pat. No. 3,486,808, Hamblen; German Pat. No. 1,913,358, Kitano, Koizumi, Matsamura; German Pat. No. 1,933,124, Uschida, Furukawa, Yoshikawa, Kitano, Koizumi; U.S. Pat. No. 3,614,197, Nishizawa, Kawakami, Kiyasu; German Pat. No. 1,901,053, Gliemeroth and Jacobsen; and German Pat. No. 2,039,239, Gliemeroth and Jacobsen.
Also, the suggestion has been advanced to start, in the production of gradient-optical elements, with homogenous inorganic glasses and generate therein refractive index gradients by ion exchange which are responsible for the optical effect, e.g., beam deflection.
Thus, for example a refractive index gradient having the shape of a parabola is produced in a homogeneous glass rod having a thickness of 10 mm. and a length of 120 mm., composed of, in percent by weight: 64% SiO.sub.2 ; 5% Li.sub.2 O; 17% Al.sub.2 O.sub.3 ; 8% PbO; 6% Na.sub.2 O, by a treatment in a NaNO.sub.3 salt bath for 164 hours at 495.degree. C.; this refractive index gradient corresponds to a maximum refractive index difference from the inside toward the outside of .DELTA. n = 85 .times. 10.sup.-.sup.4. This gradient rod, actually very suitable for image transmission, suffers from the grave disadvantage of high internal stress due to the linear thermal expansion coefficient which has been partially altered by the ion exchange. The paramount handicap of such rods is the great danger of breakage caused by even minor shocks. Other refractive index gradient systems, such as gradient fibers, for example, effected by altering the concentration, also suffer from this phenomenon, even though the optically active concentration profile was not produced by ion exchange.
Numerous investigations are found in the literature disclosing how to calculate the change in characteristics caused by a change in concentration. A summary of various techniques for calculating the linear thermal expansion coefficient and for calculating the refractive index from the concentration of the ions constituting the glass is given by H. Scholze ("Glas; Natur, Struktur und Eigenschaften" [Glass; Nature, Structure, and Properties], Friedr. Vieweg und Sohn, Braunschweig 1965). There have been many experiments over the years in science and practice corroborating the validity of such calculations.
For purposes of illustration, several examples, not pertaining to the present invention, are set forth infra to demonstrate to which extent the properties of the refractive index and the linear thermal expansion coefficient are altered by changes in the concentration of ions to be exchanged and/or by the ion exchange. Table 1 shows, with reference to known compositions outside the scope of the invention, how the refractive index and the linear thermal expansion coefficient are changed by altering the glass melt composition. Primarily, the concentration of those components was changed which can also readily diffuse as ions.
The fact that similar changes in properties can be attained by a suitable ion exchange, corresponding to the changes in properties obtained by alterations in the melt composition, can be demonstrated with reference to the changes in characteristics indicated in Table 2, produced as compared to Table 1 on the same starting compositions, but by means of ion exchange. Compositions of Table 1 were employed.
Also, this fact is disclosed in the literature. When comparing Tables 1 and 2, it can be seen that it appears to make no difference, in principle, whether the change in the concentration and the resultant changes in properties take place by altering the glass melt composition or are evoked subsequently by ion exchange.
TABLE 1 __________________________________________________________________________ Changes in Properties by Alteration of the Melt Composition Using Examples Not According to This Invention, in Percent by Weight Oxide I A I B I C II A II B III A III B IV A IV B __________________________________________________________________________ SiO.sub.2 57.67 54.61 57.67 19.80 19.80 60.00 60.00 B.sub.2 O.sub.3 -- -- -- 50.00 50.00 -- -- -- -- Al.sub.2 O.sub.3 26.69 25.27 26.69 30.00 30.00 -- -- -- -- PbO -- -- -- -- -- 66.20 66.20 20.00 20.00 Tl.sub.2 O -- -- -- -- -- 14.00 7.00 5.00 2.00 Na.sub.2 O -- 10.24 15.00 20.00 10.00 -- -- 15.00 10.00 K.sub.2 O -- -- -- -- 10.00 -- 7.00 -- 8.00 Li.sub.2 O 15.64 9.87 5.00 -- -- -- -- -- -- Linear thermal ex- pansion coefficient .times. 10.sup.7 /.degree. C. (.degree.-300.degree. C.) 118.3 104.8 97.8 108.4 107.3 105.6 137.4 108.2 118.7 Tg (.degree. C.)* 443 472 483 432 419 359 345 432 413 Refractive index n.sub.d for the wave- length of 587.6 nm. 1.5446 1.5327 1.5307 1.5002 1.4990 1.7484 1.7499 1.5512 1.5479 __________________________________________________________________________ *Tg means transformation temperature according to DIN [German Industrial Standard] 52324.
TABLE 2 __________________________________________________________________________ Changes in Properties by Ion Exchange Using Examples Not According to This Invention, in Percent by Weight Oxide I D II C III C IV C __________________________________________________________________________ SiO.sub.2 57.67 -- 19.80 60.00 B.sub.2 O.sub.3 -- 50.00 -- -- Al.sub.2 O.sub.3 26.69 30.00 -- -- PbO -- -- 66.20 20.00 Tl.sub.2 O -- -- 14.00 5.00 Na.sub.2 O -- 20.00 -- 15.00 K.sub.2 O -- -- -- -- Li.sub.2 O 15.64 -- -- -- Exchange in the medium NaNO.sub.3 KNO.sub.3 NaNO.sub.3 + KNO.sub.3 Salt Bath Salt Bath KNO.sub.3 Salt Salt Bath Bath Ion exchange temperature (*) 470.degree. C. 445.degree. C. 375.degree. C. 450.degree. C. Exchange period (*) 36 h. 48 h. 48 h. 48 h. As attained by ion exchange (*) .DELTA. n .times. 10.sup.4 35 11 15 30 ##STR1## 18.2 1.0 28.4 9.9 __________________________________________________________________________ (*) Ion exchange adapted to Tg (see Table 3). .DELTA. n and .DELTA..alpha.: see the description in the section "Measuring Methods."
Instead of the salt bath in Example IV C, it is also possible to utilize other media e.g., a solution containing K-ions at 80.degree. C. and at an exchange time of 1273 h. The result is a .DELTA.n .times. 10.sup.4 = 25 and a .DELTA..alpha..times. 10.sup.7 = 10. Compositions such as I A, for example, are moreover distinguished by a high tendency toward crystallization and thus can only be produced under technically difficult conditions.
The example of the composition I A in Table 1 is varied by the melts I B and I C with the aid of controlled changes of the mixture (Table 1) with respect to the properties of refractive index and linear thermal expansion in a direction attained on Example I D of Table 2 with the same composition as Example I A by means of ion exchange. Although an exact coincidence of the properties altered by a change in the melt composition and by means of ion exchange was not attained and was not expected, either, a comparison of the two tables demonstrates that the Examples I (according to Pearson, French, and Rawson: Appl. Phys. Letters 15 [1969]: 76) result in a glass which with a moderate change in the index of refraction has a great change in expansion. Examples II (U.S. Pat. No. 3,486,808) result in a glass which, though showing a minor change in expansion, also has a very small change in the refractive index. Examples III (German Pat. No. 1,933,124) result in a glass having a minor change in the index of refraction and a great change in the expansion; Examples IV (German Pat. No. 1,913,358 ) yield a glass having a moderate change in the refractive index and a marked change in expansion.